Many low-volume semiconductor wafers are fabricated in a single-wafer process. The fabrication process, to be profitable, must yield a certain percentage of useable wafers. In order to form a useable wafer, a process controller typically stops the process between each wafer layer when the layer under process, i.e., the active layer, which is typically the top most layer, has attained its desired thickness. The time at which a layer attains its desired thickness is the process stop point, i.e., the endpoint time for that layer. For wafers produced in a conventional process, statistical data is collected from trial runs, and this data provides the optimum process endpoint times for each layer. However, performing trial runs to collect data does not optimize the potential benefits of a single-wafer process. Therefore, semiconductor manufacturers must use other methods of predicting the process endpoint time for layers of a wafer fabricated in a single-wafer process.
Monitoring the wafer layer thickness in real time is one method used to predict the endpoint time. Typical layer-thickness monitoring systems may include a sensor, such as an interferometer. An interferometer is a device which uses waves of electromagnetic energy to obtain certain kinds of data. In layer-thickness monitoring applications, the interferometer transmits these waves toward the wafer and the wafer reflects these waves back to the interferometer. During the reflection process, the wafer layers change certain parameters of the waves, such as phase and amplitude. The type and quantity of these changes are related to the thicknesses and refractive indices of the wafer layers.
These parameter changes are processed and compared with values in a look-up table (LUT). Each value in the LUT was previously and empirically derived to correspond with a certain layer thickness. The thicknesses of the wafer layers, hereinafter the "layer thicknesses", are determined to be the values corresponding to the LUT parameter changes closest to the actual parameter changes.
The interferometer system generates from the LUT samples of the layer thicknesses at discrete time intervals. These samples are mathematically manipulated by a processor using various techniques to calculate and update a process rate, i.e., the actual rate at which the thickness of the active layer is changing. ("Updating" refers to recalculating, i.e., adjusting, the process rate each time layer thickness samples are generated.) Once the process rate is updated, the time at which the endpoint occurs for the active layer can be predicted. The processor then signals the controller to stop the process at this predicted time.
One problem with typical endpoint detection methods and systems is that the proper pre-process placement of the wafer within the processing chamber cannot be confirmed. If the wafer is not properly placed within the chamber, the electromagnetic waves from the sensor will not properly strike the wafer, and the thickness monitoring system will generate erroneous layer thickness samples.
Another problem with typical endpoint detection methods and systems is that improper processing of the previous layers cannot be detected. Thus, the current process cannot be aborted to prevent further processing of a defective wafer.
Yet another problem with typical endpoint detection methods and systems is that distortion of the transmitted or reflected waves cannot be detected. Typically, these waves enter and exit the process chamber through chamber windows. Deposits of process materials which accumulate on these windows may distort the transmitted or reflected waves. This distortion may cause parameter measurement errors which may ultimately lead to erroneous thickness samples.
Another problem with typical endpoint detection systems is that a layer thickness sample, which is out of an expected range, cannot be detected. (A faulty interferometer or rough layer surface may cause the out-of-range sample.)
Still another problem with typical endpoint prediction systems is that the calculation techniques used cannot account for trends of deviation from the ideal process rate exhibited by the actual process rate.
Out-of-range wafer thickness samples and non-accounting for trends of deviation can cause an erroneous endpoint time prediction. If this erroneous prediction is severe enough, the wafer will not meet predetermined performance standards and tolerances, and the process yield will be reduced.